Molecular biological techniques are being applied to evaluate yeast speciation and strain variability. DNA base composition, DNA reassociation, genome size, and amount of repetitive DNA will be determined for various yeasts. Medically, industrially, and ecologically important yeasts are included in this investigation. An emphasis on imperfect yeasts is made to provide necessary information on their proper identification and relationship with perfect yeasts. This research project will provide valuable contributions in the understanding of the relatedness among yeasts and to the knowledge of evolutionary trends and developments. DNA was isolated and purified from one hundred and forty-three strains of yeasts. DNA base composition (percentage GC) was determined and thirty-six DNA reassociation experiments were completed. Species and strain relatedness was evaluated for several species on the genera Candida, Torulopsis, Pichia, Hanseniaspora and Kloechera. DNA studies demonstrate that present taxonomic criteria used for species delimitation are inadequate and require revision. From a practical aspect it is expected that the data from molecular comparisons will lead to the selection of firm criteria to be used for species differentiation. Such criteria will be chosen so even simply-equipped laboratories will be able to identify yeasts with ease and precision. In the coming year, concentrated effort will continue on the DNA reassocation studies and work will begin on the other phases of the project (DNA-RNA hybridizations, genome size, repetitive DNA). BIBLIOGRAPHIC REFERENCES: Meyer, S.A., K. Anderson, R.E. Brown, M.Th. Smith, D. Yarrow, G. Mitchell and D.G. Ahearn. 1975. Physiological and DNA Characterization of Candida maltosa, a Hydrocarbon-Utilizing Yeast. Arch. Microbiol. 104:225-231. Meyer, S.A. 1975. DNA Studies of the Physiologically Similar Species, Candida tropicalis, Candida subtropicalis and Candida sake. Abstracts of the Annual Meeting of the American Society for Microbiology, p. 143.